Since M=70, then the area under the curve which is above 80 beats per minute corresponds to above + 1 standard deviation. The total shaded area corresponding  to above 1+ standard deviation  in percentage is 15.9% or  Z= 15.9/100 =0.159.  Or we can find the value of z by substituting the values  in the formula Z= X-M/ standard deviation. Therefore, Z= 70-80/10  -10/10= -1.00  is the same as +1.00. The value of z from the table for 1.00 is 0.159. How do we interpret this? This means that 15.9% of normal healthy individuals have a heart rate above one standard deviation (greater than 80 beats per minute).